1. Field of the Invention
The invention is directed to light emitting devices, and more particularly, to coatings for the facets of such devices that provide for high thermal conductivity, in part through lower thermal resistance, to enhance the transfer of heat away from the high temperature, beam emission area of the facet to improve device lifetime and reliability.
2. Related Art
Over the past fifteen years or more, much has been discussed about the passivation, hermeticity or protection of facet surfaces of laser diodes, particularly relative to the front or emitting facet. The emitting facet is also commonly referred to as the output facet of the laser diode. Passivation is the process of protecting the facet from environmental or ambient effects, particularly to oxidation, by isolating the facet from the environment. A coating is applied to the facet and its thickness is adjusted to obtain a desired level of light reflectivity at the light emitting device facet. The need to adjust the coating thickness to achieve the desired level of facet reflectivity is well known in the art. A coating may be applied to the facet surface having a thickness of xcex/4 where xcex is the laser wavelength of operation, so as to yield a low reflectivity and to enhance the lifetime of the laser. Films, such as SiO2 or Al2O3, are typically used as such protective coatings and are deposited directly on the facet surface. Also, the published art discusses problems relating to chemical stability, such as the effect of facet erosion due to the high intensity output of optical power at the facet as well as facet passivation treatment. An example of such passivation treatment is disclosed in the 1982 U.S. Pat. No. 4,337,443 of Umeda et al. entitled, SEMICONDUCTOR LASER DEVICE WITH FACET PASSIVATION FILM. In patent ""443, it is recognized that conventional passivation may not provide satisfactory protection against facet erosion attributable to photo-chemical processes that erode the facet, the result of which decreases the laser reliability. The increase in facet erosion is suppressed by the employment of an insulating film of an amorphous material that contains silicon and hydrogen as indispensable elements (xcex1-Si:H). The thickness of the coating material is in the vicinity of xcex/4n, where xcex is the laser wavelength in the material and n is the refractive index of the film, providing for maximum power output. In another patent in 1989, U.S. Pat. No. 4,815,089 to Miyauchi et al., discloses the use of a single layer of Al2O3 or SiO2 on the output facet for passivation, having a thickness preferably of xcex/3, to properly select a reflectivity in the range of 10% to 20% and provide a stabilized out-put at high powers. The concept disclosed in Patent ""089 is to select the proper thickness of the dielectric film to suppress problems relating to increased threshold, astigmatism and optical feedback noise.
In another 1989 patent to Miyauchi et al., U.S. Pat. No.4,860,305, an external cavity laser is disclosed where the rear facet is optically coupled with an external cavity for eliminating longitudinal mode hopping. The emitting facet includes a single film of Al2O3, which is likely provided for the reasons given in Patent ""089, of achieving stabilized single longitudinal mode control at higher powers in spite of aging effects.
In yet another Miyauchi et al. patent, U.S. Pat. No. 4,951,291, the emitting facet is provided with a multi-layer, dielectric coating to provide a protective coating so that oxidation of the front facet can be suppressed to attain an increase of the life span of the laser. The coating comprises a first layer of Al2O3 and a second layer of xcex1-Si:H2 which is effective for providing high reflectivity, such as 30% or less, as well as suppressing oxidation.
In the foregoing disclosures, only issues of passivation and chemical stability are addressed. Interestingly, no mention or discussion as to the effects of optical power at the facet output and its contribution to the development of high temperatures at the area of beam emission at the facet, the thermal conductivity of the facet coating and its relation to coating thickness to achieve lower thermal resistance, the causes of different photo-chemical reaction rates in facet degradation, how catastrophic optical damage (COD) occurs and can be suppressed to increase device lifetime, how thermal conductivity might be taken into account in the development of facet coatings relative to desired materials for coatings as well as coating thicknesses, and the consideration of thermal conductivity in combination with passivation and chemical stability. U.S. Pat. No. 5,422,901 to Lebby et al. employs a high thermal conductive layer in the form of diamond-like carbon (DLC) surrounding a vertical cavity laser (VCSEL) device, but does not deal with the horizontal cavity laser with an end cleaved facet having a high density output beam, i.e., there is no discussion is made of the development and employment of high thermally conductive coatings at the beam emission area to lower the device temperature at this area to enhance device lifetime and reliability. Moreover, heating is due to high current operation of the vertical cavity device and not due to optical heating of the facet due to optical absorption of a high intensity beam formed by a diffraction limited aperture provided by horizontal cavity, cleaved facet, edge emitting laser device. Also, there is no disclosure or suggestion of how to accomplish efficient heat removal from the output facet of a light emitting device with a cavity emission from a cleaved facet.
A limiting aspect of high-power single-mode and broad area light emitting devices, such as semiconductor lasers, is catastrophic optical damage (COD). COD is a thermal runaway event occurring at the emitting facet of a light emitting device. COD is a function of operational temperature of the light emitting device, the cavity width and length of the light emitting device as well as the current density and optical power density at the output facet. Facet aging leads to increased optical absorption due to surface oxidation or other chemical reactions and, ultimately, to COD which limits the lifetime or reliability of the device. Various methods for postponing the aging process by passivating the output facet have been proposed for prolonging the onset of COD as suggested by the previously discussed disclosures. High quality passivation, however, is often difficult to achieve. Moreover, absorbed optical power at the device facet is what causes a temperature rise at the facet, leading to high facet temperatures that rapidly increase in reaction rates for facet degradation mechanisms, such as chemical or photo-chemical erosion and passivation coating degradation or breakdown over time, or decrease of COD level, all due to such high facet temperatures, shortening the life time of the light emitting device. The invention herein represents an approach for achieving lower facet temperatures for increasing the COD power before, during and after facet aging, extending the life of the device through proper coating of the device facets while concurrently maintaining proper reflectivity as well as providing facet passivation and chemical stability.
Thus, it is an object of this invention to provide a coating for facets of a light emitting device that provides for a lower facet temperatures during device operation by more effectively carrying away heat developed at the facet suppressing the material onset of temperature dependent facet degrading mechanisms occurring at the facet surfaces so that higher power outputs may be achieved with improved device reliability and lifetime.
According to this invention, a light emitting device is provided with a coating that will increase the thermal conductivity at one or more facets of the device to provide for lowering the facet temperature during device operation to suppress the occurrence of temperature dependent facet degrading mechanisms affecting facet chemical stability, hermeticity and catastrophic optical damage (COD) level of the light emitting device since these facet attributes are directly affected by temperature at the facet. In the preferred embodiment, the coating should have a thermal conductivity that is higher than the material of the light emitting device. The high thermal conductivity coating provides for an efficient transfer of heat away from the beam emission area of the front facet into regions adjacent to, i.e., above or below the active region of the device, such as layers of the device underlying the active region and the device substrate. Moreover, the coating can be extended to be in direct contact with the device submount in order to possibly further enhance heat conduction away from the facet. If by coating material does not provide a sufficiently high level of thermal conductivity, then, in addition, thermal resistance should also be taken into consideration and the coating should be made thicker to obtain lower thermal resistance to achieve higher thermal transfer benefits toward lowering the facet temperature. In either case, the rate of heat transfer from the facet is enhanced so that the onset of higher temperature dependent facet degrading mechanisms developing at the device facet are reduced or suppressed. The higher the power output from the device facet, the higher the facet temperature and the prospects of early device failure due to COD. By lowering the facet temperature, for example, the onset of chemical instability developing at the facet is reduced, such as caused by chemical or photo-chemical processes. The reaction rate of such processes are reduced due to lower facet temperatures by employing the coatings of this invention. Also, COD levels are raised to new levels by the lower thermal resistance at the facet. As a result, higher output powers are achieved from the same device employing the coatings of this invention while maintaining or improving device lifetime and reliability.
As used herein xe2x80x9clight emitting devicexe2x80x9d is intended to cover small component semiconductor or polymer light emitting devices such as semiconductor laser diodes, superluminescent devices, semiconductor amplifiers and polymer-based light emitting devices. As used herein, the term, xe2x80x9ccoatingxe2x80x9d may be one or more layers or films of materials or compounds with the primary goal of achieving high thermal conductivity, such as, for example, higher than the thermal conductivity of the materials employed in the light emitting device. In the examples provided, such a high quality, high thermal conductivity may be achieved with one or more layers. In achieving additional attributes of chemical compatibility or stability and hermeticity or passivation, more than one layer may be preferably required. In all cases, the coatings set forth herein provide for optical transparency for the radiation wavelength emitted from the light emitting device. By xe2x80x9cchemical stabilityxe2x80x9d we mean a film in contact with the facet that maintains the chemical integrity of the facet and prevents facet decay due to chemical or photo-chemical reaction occurring at the facet.
In the past, coatings of facets were specifically designed with passivation and desired facet reflectivity level in mind without particular reference to those skilled in the art to thermal conductivity effects at the facet or the possible importance of efficient thermal transport of heat away from the device""s emission area to reduce facet temperature. The temperature of the facet is a very important parameter in determining chemical stability and hermeticity. As indicated previously, the prior art has remained attentive only to chemical stability or passivation. By raising the level of thermal conductivity at one or both facets of the semiconductor laser device through an applied coating that provides for a higher degree of heat spreading and high thermal transport of heat away from the beam emission area of the light emitting device, the useful operating power as well as the probable operational lifetime and reliability of the light emitting device can be increased.
A major contribution to the generation of heat at the facet emission area is the optical power density, and optical power of devices in today""s semiconductor laser devices has significantly increased over past such devices. The new mirror coating of this invention function as a heat spreading layer so that heat generated at the output facet is laterally spread across the cooler facet region immediately adjacent to the central hottest spot of the optical mode of the light emitting device so that this lateral heat spreading permits the heat to be absorbed by non-active layers of the device and the device substrate, for example. Therefore, this invention seeks and realizes a coating comprising one film or a combination of films that basically provide for high thermal conductivity either because of the facet material employed or because of its thickness to lower thermal resistance or a combination of both facet material and coating thickness. For maximum power and long term reliability of these light emitting devices, the facet coating should exhibit the three properties of high thermal conductivity, chemical stability, and hermeticity.
Thus, the high thermal conductivity of the coatings of this invention provide heat spreading at the facet emission region of the device, enabling heat to be more effectively carried away and, consequently, providing for lower facet temperature.
A first embodiment of this invention comprises a high thermal conductivity single layer applied to the device facet. Such a high thermal conductivity layer may be comprised of silicon carbide (SiC), boron nitride (BN), beryllium oxide (BeO), alumina (Al2O3), aluminum nitride (AlN), boron phosphide (BP), diamond, diamond-like carbon (DLC), boron oxide (B2O3) or magnesium oxide (MgO). The high thermal conductivity layer provides for an efficient transfer of heat away from the beam emission area of the front facet into regions adjacent to, i.e., above or below the active region of the device, such as layers of the device underlying the active region and the device substrate. Moreover, the layer can be extended to be in direct contact with the device submount in order to possibly further enhance heat conduction away from the facet.
The thickness of the layer can play an important role since thermal resistance decreases with increasing layer thickness. Thus, for optimal designed reflectivity, such as, in the range of about 0% to 30% reflectivity, i.e., to make the reflectivity easily controlled relative to the desired coating thickness, the layer thickness should be approximately in the range of xcex(4n) to xcex/(2n), where xcex is the oscillation wavelength of the light emitting device and n is the refractive index of the coating. Coating thicknesses outside this range are possible depending upon the thermal conductivity of the facet coating material. For example, a coating with a very high thermal conductivity may possibly have a useful thickness less than xcex(4n), although thicknesses greater than xcex/(2n) are not ruled out. A thickness of greater than xcex(2n) should be used for materials with somewhat lower thermal conductivity, such as alumina (Al2O3), to provide for lower thermal resistance to achieve higher thermal transfer benefits, while still maintaining the same facet reflectivity, as in the case of layers with thickness less than xcex/(2n), mentioned above. For example, a coating that is Nxcex/(4n) (where N is an odd integer greater than 1, i.e., 3, 5, 7, etc.) thick provides the same reflectivity as a xcex/(4n) layer, yet provides much lower thermal resistance. However, the greater the integer N, the more sensitive the reflectivity is to thickness and, therefore, the more difficult it is to control the reflectivity. Moreover, the greater the thickness of the coating, the more difficult is the long term adhesion of the coating to the facet due to thermal conductivity differences with the material of the laser source. As a result the coating can eventually crack and peal off of the facet. However, in cases where the thermal conductivity of the coating is not substantially greater than GaAs, for example, such as alumina (Al2O3), the advantages of heat spreading can still be obtained by employing thicker coatings, i.e., greater than xcex/(2n). Thus, DLC, having a comparatively high thermal conductivity, need not be as thick as alumina, for example. The desired thickness of layers, therefore, depends upon the thermal conductivity of the chosen coating materials.
In another embodiment of the invention, a coating comprises two or three layers providing in combination high thermal conductivity at the emitting facet as well as hermeticity and chemical stability in the case where a single layer comprising the coating does not substantively provide all these properties. A first approach for a two layer coating comprises a first transition layer of alumina (Al2O3), GaP, ZnSe, or the like, providing good chemical stability on the out-put facet, such as in the case of output facets of an AlGaInP or AlGaAs laser diode. In one embodiment, a first transition layer in contact with the facet provides for chemical stability and has a thickness sufficient to provide good chemical stability, which thickness is in the range of about 20 xc3x85 to about 2,000 xc3x85. A second, thermal transfer layer provides for high thermal conductivity and hermeticity and may have a thickness greater than approximately xcex/(4n). The material for the second layer, transparent to the device light, may be comprised of high thermal conducting materials, such as, SiC, BN, BeO, AlN, BP, DLC, B2O3 or MgO or the like. The thickness of the second layer is preferably in the range of about 1,000 xc3x85 to about 5,000 xc3x85, but optimum thickness is also dependent upon the chosen method of deposition of the material and is somewhat dependent upon the thermal conductivity of the material employed for this layer.
In a second approach for a two layer coating, a first layer may exhibit both properties of chemical stability and good thermal conductivity, while the second layer may also have the quality for hermeticity. An example of such a layer combination is a thick layer Al2O3 (i.e., thickness greater than xcex/2n) and a second layer of HfO2, SiNx, SiC, SiO2, ZrO2 or TiO2, which may be sufficiently thick to provide for good hermeticity, such as, for example, having a thickness in the range of about 20 xc3x85 to about 2,000 xc3x85. The exact thickness of the outer layer of HfO2, SINx, SiC, SiO2, ZrO2 or TiO2 is also adjusted to achieve the desired level of reflectivity.
It is possible to provide for a high thermal conductivity coating with any arbitrary number of layers providing the basic benefits of thermal conductivity, chemical stability and hermeticity, but the additional complexity of more than three layers is generally to be avoided from the standpoint of additional costs and manufacturing complexity in depositing multiple layers as well as the complexity of maintaining the desired level of reflectivity at the facet, which is generally 30% or less.
For all of the foregoing embodiments, the exact, desired reflectivity must be obtained by proper adjustment of the layer thickness. With increasing number of layers employed for a coating, control of the desired level of reflectivity becomes more difficult. Therefore, it is preferred to employ a high thermal conductive coating with a minimum amount of layers, the optimum being a single layer.
The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of several preferred embodiments of the invention, as illustrated in the accompanying drawings.